A MICS/MEDS based system includes at least a medical controller, also known as hub, and a medical implant, also known as node, operating in the MICS band 402-405 MHz or the MEDS band 401-402 MHz and 405-406 MHz. The medical implant is located inside the body of a living organism and the medical controller is, in most cases, externally located. Medical devices may be implanted within the human body for medical diagnosis and treatment. These medical implants are miniaturized devices that typically operate as sensors or stimulators. Because it is located within a patient's body, it is impractical for a doctor, medical professional, technician, or monitoring and control device to communicate by direct, physical connection with the implant. Instead, wireless communications are used to establish links between the implant and external equipment and medical professionals. The wireless communications provide fast and reliable data exchange with the implant without requiring a direct physical connection to the patient, which allows the patient more freedom and less discomfort. External monitoring and control equipment may communicate with the implant at the patient's home, in a doctor's office, in a hospital, or at other locations. The power consumption by the transceiver forms a significant portion of the overall power consumption in the medical implant. Hence, it is desired to maximize efficiency of the medical implant transceiver to increase lifetime of the medical implant.
The location of the implanted medical device makes power consumption a critical parameter. The implant may be powered by an internal battery that is recharged periodically, such as by inductive radio frequency (RF) links. In other embodiments, medical regulations may require surgical replacement of an implant device that has low battery power, which makes low-power operation of the node a critical factor. It is important to achieve a balance between low power consumption of the implant and high quality wireless communication between the implant and external equipment. It is often not practical or even possible to reduce power consumption by simply minimizing the frequency with which the implant and external equipment communicate. Many implants require the capability to send and/or receive information with the external equipment with minimal delay at any time. For example, at any time, conditions may arise that require an implanted stimulator to change operating parameters, or an implanted sensor may need to report a critical or emergency condition. However, it is not acceptable for the implant to maintain a continuous wireless link to the external equipment because of the power required to maintain such a connection and the subsequent drain on the implant's battery or power supply.
The power consumption in the medical implant is dominated by the search for communication with an external controller in both an unconnected state and a connected state of the medical implant. In the unconnected state, the medical implant wakes up periodically, such as every few seconds, and listens for a signal for connection with an external controller. In a connected state, the medical implant follows a procedure similar to that for the unconnected state, and wakes up periodically, which may be every few seconds, and listens for the external controller with which it has connected. In one example, the medical implant in a connected state follows the procedure because the medical implant needs to detect another medical controller. For example, if the medical implant is initially connected with a medical controller at home, but the living organism containing the medical implant travels to a doctor's office, then the medical implant needs to detect and connect with the medical controller at the doctor's office. Hence the medical implant wakes up periodically, even when the medical implant is already connected with a medical controller, to detect other medical controllers. However, waking up in the connected state with a periodicity similar to that of the unconnected state leads to power wastage when the medical implant has to search for signals that typically have low strength. As the time for which the medical implant has to listen for low-strength signals increases, more power is consumed.
Communications between a controller (or “hub”) and an implant (or “node”) are subject to stringent regulatory restrictions. In particular, the hub often has to switch to a new operating channel selected from the ten channels total in the MICS band. The node then needs to discover that new operating channel to communicate with the hub. A node cannot normally initiate a transmission to the hub, unless in an “implant report event” (i.e., medical emergency). Furthermore, the node cannot choose the operating channel to use in an emergency, but instead the node must discover the hub's current operating channel to report the emergency. Existing solutions are either not power efficient or not in compliance with the FCC requirements for the MICS band. There is a need for a new technique for supporting power-efficient communications between implants and controllers.